The present invention relates to a solid-state image pickup device, and more particularly to a CCD linear image sensor for color reading used in color copiers, color image scanners or the like.
Recently the demand for color representation is rapidly mounting in copiers, image scanners and other appliances, and CCD color linear image sensors playing an important role of reading images in these appliances are expected to be enhanced in performance.
Hitherto, the CCD color linear image sensor of this type generally had the structure as disclosed in the publication 5 k-bit .times.3 (R, G, B) color line sensor for reduction optical system (Fujii, Kadowaki: Hitachi Review, Vol. 72, No. 7). This structure and the image reading method are explained below while referring to FIG. 5 and FIG. 6.
As shown in FIG. 5, the conventional CCD color linear image sensor comprised three CCD linear image sensors for reading red, green and blue colors disposed adjacently in a single chip, consisting of photodiode lines consisting of photodiode lines (11a, 12a, 13b, . . . , 1na), (11b, 12b, 13b, . . . , 1nb), (11c, 12c, 13c, . . . , 1nc), shift gates 3a, 3b, 3c for signal charge transfer disposed parallel to these photodiode lines, CCD analog shift registers for signal charge transfer (21a, 22a, 23a, . . . 2na), (21b, 22b, 23b, . . . 2nb), (21c, 22c, 23c, . . . 2nc), charge-voltage converters 4a, 4b, 4c, and output terminal 5a, 5b, 5c. The photodiode lines (11a, 12a, 13a, . . . 1na), (11b, 12b, 13b, . . . 1nb), (11c, 12c, 13c, . . . , 1nc) are provided with color filters of, for example, red, green and blue, in every line for color separation.
The individual photodiodes for composing the photodiode lines (11a, 12a, 13a, . . . 1na), (11b, 12b, 13b, . . . 1nb), (11c, 12c, 13c, . . . 1nc), are generated at pitches p, and the interval of photodiode lines (11a, 12a, 13a, . . . 1na) and (11b, 12b, 13b, . . . 1nb) is L1, and the interval of photodiode lines (11b, 12b, 13b, . . . 1nb) and (11c, 12c, 13c, . . . 1nc) is L2.
The photogenerated signal charges in the photodiode lines (11a, 12a, 13a, . . . , 1na), (11b, 12b, 13b, . . . 1nb), (11c, 12c, 13c, . . . , 1nc) are transferred through shift gates 3a, 3b, 3c respectively to CCD analog shift registers (21a, 22a, 23a, . . . 2na), (21b, 22b, 23b, . . . , 2nb), (21c, 22c, 23c, . . . , 2nc). The signal charges transferred to the CCD analog shift registers (21a, 22a, 23a, . . . , 2na), (21b, 22b, 23b, . . . , 2nb), (21c, 22c, 23c, . . . , 2nc) are sequentially transferred to the charge-voltage converters 4a, 4b, 4c by applying clock pulses, and are produced from the output terminals 5a, 5b, 5c as voltage changes.
By the CCD color linear image sensor based on the prior art having the above constitution, the optical system for reading color images is composed approximately as shown in FIG. 6. The light reflected by a document 6 is reduced to 1/m through a lens 7, and is projected on three photodiode lines of the CCD color linear image sensor. In this case, since the interval of the photodiode lines (11a, 12a, 13a, . . . , 1na) and (11b, 12b, 13b, . . . , 1nb) is L1 and the interval of the photodiode lines (11b, 12b, 13b, . . . , 1nb) and (11c, 12c, 13c, . . . , 1nc) is L2, the information on the straight line at a distance of L1.times.m between the photodiode lines (11a, 12a, 13a, . . . , 1na) and (11b, 12b, 13b, . . . , 1nb) or at a distance of L2.times.m between the photodiode lines (11b, 12b, 13b, . . . , 1nb) and (11c, 12c, 13c, . . . , 1nc) is read on the document surface.
Supposing the document is moving at a specific speed v in the direction of the arrow in order to scan the whole document, with respect to the information on the same straight line on the document, the signal of the photodiode line (11c, 12c, 13c, . . . , 1nc) is obtained prior to the signal of the photodiode line (11a, 12a, 13a, . . . , 1na) by the time T.sub.c =(L1+L2).times.m.div.v, and the signal of the photodiode lines (11b, 12b, 13b, . . . , 1nb), prior to the signal of the photodiode line (11a, 12a, 13a, . . . , 1na) by the time T.sub.b =L1.times.m.div.v.
In the CCD color linear image sensor conforming to such prior art, in order to obtain the color information on the same straight line on the document in signal processing, it is necessary to synthesize the signals of three photodiode lines by delaying the signal of the photodiode line (11c, 12c, 13c, . . . , 1nc) and the signal of the photodiode line (11b, 12b, 13b, . . . , 1nb) against the signal of the photodiode line (11a, 12a, 13a, . . . , 1na) by the time T.sub.c and the time T.sub.b, respectively.
Generally, to realize such signal delay time, for example, a semiconductor digital memory device is used. In the CCD color linear image sensor according to the prior art, the density of integration of CCD analog shift registers placed between the photodiode lines is limited, and it is very difficult to set the photodiode line intervals L1, L2 under 12p. Using a CCD color linear image sensor having 5,000 photodiodes each in photodiode lines with the photodiode line intervals L1, L2 defined as L1=L2=12p, the horizontal and vertical directions on the document are read at the same reading resolution. When the signals of three lines of photodiodes are digitalized into 8-bit information individually, the memory capacity necessary for the semiconductor digital memory device for delay necessary for time axis correction of photodiode lines is 5,000 bits.times.(12+24).times.8=1,440,000 bits, that is, about 1.4 Mbits, and when using such a CCD colorlinear image sensor, the color copier or color image scanner may be notably increased in cost and complicated in circuit composition.
The invention is intended to solve such problems, and it is a primary object thereof to present a CCD linear image sensor capable of significantly simplifying the signal time axis correction circuit of a semiconductor digital memory device or the like.